DE10020951A1 - Optic dispersion compensator, preferably before or after optic transmission path - Google Patents

Abstract

The optic dispersion compensator includes at least one optic input (15), at least one frequency demultiplexor (FDM, 1) which divides incoming signals with an input spectrum into two frequency ranges (fL and fH), and two transmission paths (4.1, 4.2) of different optical length, which are respectively supplied with one frequency range. The optically longer transmission path serves as delay conductor (4.2). At least one frequency recombination unit (3) recomposes both spectrally divided signals, and provides them at, at least one optic output (16). A polarisation converter (6) is provided in at least one transmission path.

Description

The invention relates to an optical dispersion compensator, preferably for use in front of an optical transmission path or following an optical transmission path, with at least one optical input, at least one frequency demultiplexer (FDM), which incoming signals with an input spectrum in two frequency bands f _L and f _H disassembled and two transmission links (Mach-Zehnder arms) of different optical lengths, each of which a frequency band (f _L , f _H ) is fed, with the optically longer Mach-Ten der-Arm serving as a delay line, and then at least a frequency recombination unit in which the two spectrally decomposed signals are recombined and routed to at least one optical output.

The invention also relates to a method for Dispersionskom compensation of an optical signal transmitted via a glass fiber with a frequency spectrum composed of two frequency bands f _H , f _L , the frequency bands being split into one Mach-Zehnder arm, experiencing different delay times and then again be brought together

When transmitting optical signals of a certain Frequency spectrum (of a certain bandwidth) over a optical fiber of great length, for example an optical fiber, it comes because of the frequency-dependent Ge speed of light in the glass fibers to disperser phenomena, i.e. a distortion of the input light pulse se / input bit sequence depending on the path length. This chromatic dispersion of the glass fibers limits the maximum bridgeable distance in the high bit rate transmission systems stemen. For example, the common singlemode  Glass fibers with a dispersion of 17 ps / nm.km at a Wavelength 1550 nm a bridgeable distance of only 80-100 km in 10 Gbit / s systems without dispersion compensation. Every further doubling of the transmission bandwidth is reduced roughly bridges the maximum distance that can be bridged by a factor of 4. For longer transmission distances, the dispersion of the Glass fiber can be compensated accordingly.

The previously known methods for dispersion compensation are due to their importance for high bit rate transmissions systems are very numerous. They can be roughly electro Classify niche and optical compensation techniques.

One finds among the electronic compensation procedures first the pre-chirp techniques. They are based on the ore a negative frequency chirp of the laser diode and enable appropriate precompensation. Furthermore can be by suitable modulation methods such as single sideband modulation, duobinary modulation, etc. a reduction in input bandwidth with the same bit rate aims and thus increases the maximum distance that can be bridged become.

The electronic compensation techniques are general quite cumbersome and their implementation depends on that too transmitting bit rate. Another problem is that electronic compensation techniques are not optically trans are parents.

In the optical compensation process, an attempt is made to Dispersion of the transmission path by an appropriate opposite dispersion of the optical compensation element to reproduce mentes as completely as possible. With optimal Replica of the dispersion, including the dispersion therme of higher order, can be neglected if the nonlinear  Effects potentially full compensation be achieved.

Special dis Persistence Compensating Fibers - DCF's) developed in the optical transmission system men are now largely used. Here goes through one Bit sequence either before or after the actual dispersive one Transmission route section the appropriately dimensioned DCF. With the dispersion values of the DCF's are needed to compensate for 100 km of transmission stretch a DCF length of un over standard single-mode fibers about 15 km.

These DCFs are optically transparent and allow one Multi-channel compensation, but they suffer from their low level Compactness, have a not insignificant damping and no adjustable dispersion. It must therefore be for everyone Transmission length the length of the DCF accordingly fit what additional logistical problems with it brings.

Another optical compensation technique is based on the "ge chirpten "Bragg Gratings (fiber optic or integrated op realized table). The "chirped" Bragg gratings are indeed some more compact than the DCF's, but they work in Re flexion and must therefore be combined with a circulator the. The dispersion bandwidth of a gratification is also limited and each individual wavelength channel must be separate be compensated. Furthermore, over a wide dis adjustable Bragg gratings not easy realize, also because of the compensation bandwidth and the The reflection coefficient depends on the set dispersion gene.

Another option is dispersion compensation circuits integrated optically in planar technology,  fiber-optic or volume-optical with the help of interferomes trical configurations. The interferometri configurations are based on the use of asymme trical Mach-Zehnder interferometers, ring resonators or the Fabry-Perot resonators.

Regarding the compensation techniques described above Mach-Zehnder is on the writings

• - K. Takiguchi, K. Okamoto and K. Moriwaki, "Planar Lightwa ve Circuit Dispersion Equalizer ", J. Lightwave Technol., vol. 14, pp. 2003-2011, 1996;
• - K. Takiguchi, S. Kawanishi, H. Takara, A. Himeno. K. Has tori, "Dispersion Slope Equalizer for Dispersion Shifted Fiber Using a Lattice-Form Programmable Optical Filter on a Planar Lightwave Circuit ", J. Lightwave Technol., vol. 16, pp. 1647-1656, 1998; and
• - K. Jinguji, M. Kawachi, "Synthesis of Coherent Two-Port Lattice-Form Optical Delay-Line Circuit ", J. Lightwave Technol., Vol. 13, pp. 73-82, 1995;

regarding ring resonators and Fabry-Perot

• - C. K. Madsen, G. Lenz, 'Optical All-Pass Filters for Phase Response Design with Applications for Dispersion Compensati on ", IEEE Photon. Technol. Lett., vol. 10, pp. 994-996, 1998 referenced and their full disclosure content via taken.

The technical problem of the above interferometric Structures is that they are only one without cascading very limited dispersion compensation with ge enable a wide dispersion bandwidth. The one here necessary cascading inevitably leads to egg increasingly difficult to implement and more complex Structure.

It is therefore an object of the invention to provide a dispersion compen sator and a method for dispersion compensation to fin which high dispersion values at the same time  high bandwidth without cascading multiple filter stages com can retire. The invention is also intended to make it possible to generate adjustable dispersion values.

The task is solved by the two independent patent claims che 1 and 2 solved.

With this invention, the inventor proposes a structure which has a high dispersion compensation with any size Dispersion bandwidth without cascading multiple filters levels. It is based on the two partial signals an asymmetrical Mach tenth despite the existing coherence merge without interference. This is then possible Lich if the two signals at the signal recombination to are mutually orthogonally polarized.

According to this inventive idea, the inventor proposes an optical dispersion compensator, preferably for use in front of an optical transmission link or following an optical transmission link, with at least one optical input, at least one frequency demultiplexer (FDM), which incoming signals with an input spectrum in two Frequency bands f _L and f _H disassembled and two transmission lines (Mach-Zehnder arms) of different optical length, each of which a frequency band (f _L , f _H ) is fed, with the optically longer Mach-Zehnder arm serving as a delay line , and then to improve at least one frequency recombination unit in which the two spectrally decomposed signals are recombined and led to at least one optical output in such a way that a polarization converter is provided in at least one Mach-Zehnder arm. For structural reasons, this is preferably the Mach-Zehnder arm with the shorter optical length.

According to the invention, this dispersion compensator can both as well as after an optical data transmission route be brought.

An advantageous embodiment of the dispersion compensator provides that the multiplexer in the form of a TE / TM Polarisa tion combiner is realized. This is possible if the two orthogonally supplied recombination signals according to the respective main axes (TE and TM) are polarized. In in this case the two signals are combined theoretically without 3 dB power loss (3 dB power penalty) instead of. Alternatively, a 3 dB coupler can also be used which, however, then causes the aforementioned loss of performance.

In addition, the dispersion compensator can be designed in this way be that at least one Mach-Zehnder arm, preferably the Arm with the longer optical length and / or without Polarisa tion converter, in at least two sections (in general NEN case is split into N sections), with an on controllable 1 × N switch, a controllable N × 1 switch and N sections are provided between the switches. Here is achieved by that the delay time of Teilfre quenzband and thus the achievable dispersion adjustable becomes.

In this embodiment of the dispersion com pensators can, without excluding other variants, the 1 × N switch and N × 1 switch, for example, thermo-optical or operated electro-optically.

If there is no linearly polarized at the input of the compensator Polarization state is present or a linearly polarized Input condition is present, which is not with the main axis matches the waveguide of the compensator, and at the same time, the waveguide of the compensator is anisotropic are formed, the two signals are different  Frequency bands no longer when merging or thogonal polarized.

In order to regain orthogonality in this case further in a special embodiment of the invention suggested that in at least one transmission link the Mach-Zehnders a quickly adjustable TE / TM phase shift Above, preferably located behind the polarization converter is not. This can be achieved by suitable control of the TE / TM phase shifter in the case of anisotopic waveguides Orthogonality of the recombination signals when combined leadership must be ensured.

Should the dispersion compensator after an optical signal distance can be used, so it can make sense or even he be required to go into the dispersion compensator to linearize the state of polarization. This can be done by Use of a polarization controller in front of the dispersion comm pensator happen. This polarization controller can through an input-side TE / TM divider and an output frequency recombination unit (multiplexer) reali with one of the two Mach-Zehnder arms a polarization converter and one of the two mach Zehnder with a quickly adjustable phase shifter is equipped.

In another embodiment of the signal path, the Polarization controller with a bipolar butt on the input side larization converter and mode sorter and an output frequency recombination unit (multiplexer) reali siert, whereby here also one of the two Mach-Zehnder arms quickly adjustable phase shifter.

When attaching the dispersion compensation element after the Transmission path still exists before the compensator a fast polarization scrambler and then attach a TE mode or TM mode polarizer.  This also enables a homogeneous linear Input polarization state in the compensator, an additional 3 dB power penalty must be accepted.

Furthermore, the inventor proposes according to his inventions also a method for dispersion compensation an optical signal transmitted via a glass fiber a frequency spectrum composed of two frequency bands but fH, fL, the frequency bands each being a Mach ten the arm split, different delay times experienced and then brought together again, which is characterized in that the two frequency bands which polarizes orthogonally to each other when merging are.

It should also be noted that the compensator is fiber optically, volume-optically and / or integrated optically reali can be settled. It goes without saying that make sure that the components used are no additional Generate twists in the polarization states.

Other features and special embodiments of the Er invention result from the subclaims and the successor ing description of the exemplary embodiments with reference on the drawings.

The invention will be described in more detail below with reference to the drawings described:

Fig. 1: Elementary functions of an (integrated optical, fiber optic or volume optical) dispersion compensator, based on a frequency demultiplexer, an asymmetrical Mach-Zehnder and a frequency recombination unit (= multiplexer) (prior art);

Fig. 2: Dispersion compensation asym metrical by cascading Mach-Zehnder interferometer (prior art);

Fig. 3: dispersion compensation in analogy to Figure 1, but additionally with the installation of a polarization converter in one of the two interferometer arms;

Fig. 4: dispersion compensation as shown in Figure 3, but with adjustable delay line for the f _H - spectral band;.

Fig. 5: dispersion compensation as in Figure 4, but expanded by a TE / TM phase shifter to compensate birefringent waveguides with any homogeneous elliptical input polarization state;

FIG. 6a: dispersion compensation as shown in Figure 4, but with upstream polarization controller;.

Fig. 6b: Another version of the dispersion compensation as in Figure 4 with an upstream polarization controller;

Fig. 7: dispersion compensation as in Fig. 4, but expanded by an upstream polarization scrambler with subsequent TE polarizer.

Fig. 1 shows schematically a known arrangement for dispersion compensation using an asymmetrical Mach-Zehnder interferometer with an input 15 and two outputs (Output1 and Output2) at 16. The elementary functions necessary for dispersion compensation, such as the spectral decomposition of the signal with With the aid of a frequency demultiplexer (= FDM = Frequency division multiplexer) 1 , the spectrally dependent time delay ΔL (Δτ) and the signal recombination in the multiplexer 3 are implemented in an integrated implementation on a common output substrate.

In this example according to FIG. 1, the input-side frequency spectrum F _L , f _{H of} the signal by the FDM 1 is divided into two frequency bands f _H (f _H = high frequencies) and f _L (f _L = low frequencies = low frequencies ) divided between the two Mach-Zehnder arms 4.1 and 4.2 . As also shown in FIG. 1, for the exact phase adjustment in one of the interferometer arms an adjustable phase shifter 2 is additionally necessary for adjusting the phase difference Δϕ, which may be based on the thermo-optical or electro-optical effect. The separated signals are then brought together again via a multiplexer 3 .

This configuration represents a filter stage. The phase relationship of the waves interfering in the multiplexer 3 must not change too much depending on the frequency in order to be able to realize the desired compensation bandwidth without large intensity and time delay ripples. However, this requirement limits the maximum achievable delay time Δτ and thus the dispersion per filter stage.

As a result, large dispersion compensation over a large fire range can only be achieved by cascading several asymmetrical Mach tens (filter stages). Such an implementation which is customary in the prior art is shown in FIG. 2. Here the individual asymmetrical Mach-Zehnder interferometers are connected to one another by directional couplers 5 and at the same time fulfill the functions of frequency multiplexing, frequency-dependent delay and frequency demultiplexing.

The cascading creates a successive coherent overload the wave components of the two interferometer arms testifies. The greater the desired dispersion compensation at the same time there is wide bandwidth, the more cascading levels are required. The realization is therefore increasing mend more difficult, especially since the optical path length respectively the phase of each interferometer or one each filter stage must be checked exactly. This  can be done by a thermo-optical phase shifter hen. Configurations with almost any adjustable comps values, possibly through adjustable couplers conceivable, but increase the complexity again.

Fig. 3 shows an inventive construction of an optical dispersion compensator in analogy to Fig. 1, but in addition a polarization converter 6 in the Mach-Ten der-Arm 4.1 (interferometer arms) is used. The aim is to achieve a signal recombination of two frequency bands without causing interference due to their orthogonal polarization states. By appropriate dimensioning of ΔL (= length difference of the Mach-Zehnder arms), arbitrarily large time delays Δτ of the frequency band f _H can be achieved, and this with an arbitrarily large total bandwidth of the signal f _L + f _H.

The arrangement initially consists of the frequency demultiplexer (FDM) 1 , which divides the input spectrum into two frequency bands f _L and f _H. Ideally, the FDM 1 has a rectangular frequency response, ie the edges fall off as steeply as possible. In the following asymmetrical Mach-Zehnder interferometer, the two frequency bands f _L and f _H experience a different delay time. In the case of isotropic and at the same time polarization-maintaining waveguides, as is basically possible, for example, with an integrated optical implementation, the polarization input state, as shown in FIG. 3, can have any ellipses. The polarization states are indicated by the formation of the ellipses shown. The polarization converter then converts the signal of the Mach-Zehnder arm 4.1 from an arbitrarily elliptically polarized state into an orthogonal elliptically polarized signal. This signal is then combined with the time-delayed signal from the Mach-Zehnder arm 4.2 in the multiplexer 3 .

This multiplexer 3 , in which the two frequency bands are brought together and superimposed again, can in a simple implementation, for example, consist of a broadband 3 dB coupler, as a result of which an additional power loss of approximately 3 dB is recorded. In this case, an output of the multiplexer 3 can be used as a monitor output. This can be used to monitor the output power.

If, as shown in FIG. 3, the configuration consists of isotropic waveguides, any desired elliptical polarization state of the input signal is permissible for its correct functioning, the ellipse should be as identical as possible over the entire channel bandwidth. In the case of a linear and axis-identical input polarization state, the multiplexer can be implemented by a TE / TM polarization combiner, which enables the signals to be combined without a 3 dB power loss.

The dispersion of the transmission link is weak or non-dispersive waveguides due to the two-stage time hesitation only roughly approximated, but this is an essential Chen can improve the signal. By suitable Dimensioning of the delay line ΔL can be any big large two-stage time delay Δτ without cascading can be achieved, which is particularly the case with large compensation units a significant simplification. Furthermore the arrangement comes without an adjustable phase shifter out. This means that the configuration requires a permanently set FDM, polarization converter and multiplexer no other Readjustment.

If the dispersion of the transmission link is ideal can be tried, in the Mach-Zehnder- Poor dispersive waveguides to be specially developed for this purpose to use.  

FIG. 4 shows a variant of the structure shown in FIG. 3 with an adjustable time delay. For this purpose, two adjustable switches 7 and 8 are inserted into the Mach-Zehnder arm 4.2 , which, depending on the control, the f _H spectral band to a corresponding delay line of different lengths 4.2.1 . , , , 4.2.N puts. The switches can be controlled, for example, thermo-optically or electro-optically. In this arrangement, too, a possible phase shifter for exact phase adjustment is not necessary.

Analogously to FIG. 3, the configuration shown in FIG. 4 ideally requires either an identical, homogeneous input polarization state over the entire channel frequency bandwidth and at the same time isotropic waveguides, or a linear TE or TM input polarization state with any anisotropic waveguides. The configurations according to FIGS. 3 and 4 are therefore particularly recommended for dispersion compensation before the transmission path, for example directly after the transmission laser with its defined, linear Polarisationszu.

Can be the arrangement of FIG. 4 ladders with anisotropic waves realized, so at any homo genes elliptical input polarization state, the use of an adjustable TE / TM phase shifter 9 is in one of the Mach Zehnder arms in the correct function of the dispersion compensator necessary. This can, as shown in FIG. 5, be arranged for example after the polarization converter 6 and ensures the orthogonality of the two interferometer signals in the Mach-Zehnder arms 4.1 and 4.2 when they are brought together. The TE / TM phase shifter 9 must compensate for the accumulated anisotropy difference between the two Mach-Zehnder arms.

For the general case of anisotropic Disper waveguide sion compensator can also the input polarization state  through an additional upstream polarization controller be rotated so that it is linearly polarized and the same in time with one of the waveguide axes of the dispersion comm pensators matches.

Such an embodiment can be indicated in particular when using the dispersion compensator according to the invention after a transmission path. In addition to the designs known in the literature, FIGS . 6a and 6b show two possible configurations for this.

The polarization controller 17 of the arrangements according to FIGS. 6a, 6b regulates the polarization state so that it coincides with the main axes of the waveguides of the subsequent dispersion compensator 18 (TE or TM polarization). The main axes of the waveguide of the subsequent dispersion compensator 18 may thus have any anisotropy. The polarization controller 17 of FIG. 6a, 6b can simultaneously serve a limited extent to compensate for the Polari sationsmodendispersion.

As a further variant for an application of the configurations shown in FIGS . 3 and 4 after the transmission path, the use of an upstream fast polarization scrambler 13 with subsequent TE or TM polarizer 14 would be conceivable. The waveguides 4.1 , 4.2 of the dispersion compensator 18 may have any anisotropy in this case, since the input polarization state of the light wave is oriented in one of the main axial directions (TE or TM polarization) by the upstream polarizer 14 . The polarization scrambler 13 is used either at the line entrance or directly in front of the compensation element with the upstream polarizer. The polarizer should always be placed directly in front of the actual compensator element.

As an example, FIG. 7 shows a configuration with a polarization scrambler 13 and TE polarizer 14 connected upstream of the dispersion compensator 18 .

In this configuration with polarization scrambler and on closing polarizer must an additional power ver 3 dB are acceptable.

All configurations described can, depending of the nature of the transmission path, for on channel or multi-channel compensation can be used. The rea Configuration can either be integrated op table (on a common substrate in integrated or hybrid form), fiber optic or with the help of micro optics (volume optical) components.

The invention thus describes a method for dispersion compensation and a dispersion compensator for carrying out the method, wherein an optical signal is split into two frequency bands f _H and f _L and on two Mach-Zehnder arms, where he runs different delay delays and the Frequency bands are then brought together again and are polarized orthogonally to one another.

Overall, this invention makes a dispersion compensation gate and a method for dispersion compensation which high dispersion values high bandwidth at the same time without cascading several files levels can compensate. It also enables Invention to also generate adjustable dispersion values.

Claims (12)

1. Optical dispersion compensator, preferably for use in front of an optical transmission link or following an optical transmission link, with at least one optical input ( 15 ), at least one frequency demultiplexer (FDM) ( 1 ), which signals with an input spectrum in two signals Frequency bands f _L and f _H broken down and two transmission links (Mach-Zehnder arms) ( 4.1 , 4.2 ) of different optical lengths, each of which a frequency band (f _L , f _H ) is fed, the optically longer Mach-Zehnder arm than Delay line ( 4.2 ) is used and then at least one frequency recombination unit ( 3 ) in which the two spectrally split signals are recombined and routed to at least one optical output ( 16 ), characterized in that a polarization converter ( 6 ) is provided. 2. Dispersion compensator according to the preceding claim 1, characterized in that the frequency recom bination unit as a TE / TM polarization combiner or 3 dB coupler is formed. 3. Dispersion compensator according to one of the preceding claims 1 to 2, characterized in that at least one Mach-Zehnder arm ( 4.2 ) is split into at least two sections (4.2.1... 4.2.N), a controllable 1 × N -Switch ( 7 ), a controllable N × 1 switch ( 8 ) and N sections (4.2.1... 4.2.N) are provided between the switches. 4. Dispersion compensator according to the preceding claim 3, characterized in that the 1 × N switch ( 7 ) and the N × 1 switch ( 8 ) have a thermo-optical or electro-optical control. 5. Dispersion compensator according to one of the preceding claims 1 to 4, characterized in that in at least one Mach-Zehnder arm ( 4.1 ) a TE / TM-Pha senschieber ( 9 ), preferably behind the polarization converter ( 6 ), is provided. 6. Optical signal path, characterized in that a dispersion compensator ( 18 ) according to one of the preceding claims 1 to 4 is provided and a polarization controller ( 17 ) is connected upstream of the dispersion compensator ( 18 ). 7. Optical signal path according to the preceding claim 6, characterized in that in the polarization actuator ( 17 ) two Mach-Zehnder arms ( 19.1 , 19.2 ) are easily seen, in at least one of the Mach-Zehnder arms ( 19.1 , 19.2 ) a phase shifter ( 9 ) is provided. 8. Optical signal path according to the preceding claim 7, characterized in that the two Mach-Zehnder arms ( 19.1 , 19.2 ) of the polarization controller ( 17 ) between an input-side TE / TM divider ( 11 ) and an output-side frequency recombination unit (multiplexer) ( 3 ) are arranged. 9. Optical signal path according to the preceding claim 8, characterized in that at least one of the Mach-Zehnder arms ( 19.1 , 19.2 ) of the polarization controller ( 17 ) with a polarization converter ( 6 ) is permitted. 10. Optical signal path according to claim 7, characterized in that the at least two Mach ten of the arms ( 19.1 , 19.2 ) of the polarization controller ( 17 ) between an input-side bipolar polarization converter and mode sorter ( 12 ) and an output-side frequency recombination unit ( Multiplexer) ( 3 ) are arranged. 11. Optical signal path, characterized in that a dispersion compensator ( 18 ) is provided according to one of claims 1 to 4, which is provided on the input side a polarization scrambler ( 13 ) and a TE or TM mode polarizer ( 14 ). 12. A method for dispersion compensation of an optical signal transmitted via a glass fiber with a frequency spectrum composed of two frequency bands f _H and f _L , the frequency bands being split onto a Mach-Zehnder arm ( 4.1 , 4.2 ), experiencing different delay delays and then are brought together again, characterized in that the two frequency bands are polarized orthogonally to one another when they are brought together.